The present invention relates to a developing device which makes developer adhere onto a photoreceptor which supports a latent image, and makes the latent image a visible image.
In developing devices which develop a latent image supported on a photoreceptor, developing devices which convey a two component developer (hereafter, merely referred to as developer) composed of toner and carrier by a developing roller constituted with a developing sleeve, which incorporates a magnet roller therein, to a facing portion to face the photoreceptor so as to make toner adhere onto a latent image on the photoreceptor and to make the latent image a visible image, are well known. Further, in order to obtain an image with high quality at high speed, a technique to convey developer with a plurality of developing rollers so as to develop a latent image is used (for example, Patent Document 1).
In the development by the plurality of developing rollers, first, toner is made to adhere onto a latent image on a photoreceptor at a developing region being a facing position between a developing sleeve (hereafter, referred to as an upstream side developing sleeve) of a developing roller (hereafter, along the direction in which developer is conveyed, referred to as an upstream side developing roller) which supports and conveys agitated developer, and a photoreceptor, and then the developer which have passed the developing region and remains on the upstream side developing sleeve is handed over to a developing sleeve (hereafter, referred to as a downstream side developing sleeve) of a developing roller (hereafter, along the direction in which developer is conveyed, referred to as a downstream side developing roller). The downstream side developing sleeve conveys the received developer to a developing region which is a facing portion between the downstream side developing sleeve and the photoreceptor and makes toner adhere onto the latent image on the photoreceptor.
As compared with the development by a single developing sleeve, high developing efficiency can be obtained with the development by the plurality of developing rollers. The developing efficiency can be increased with the development by the upstream side developing roller which supports and conveys agitated developer, and supplements the development by the upstream side developing roller with the development by the downstream side development, whereby high quality images at high speed can be realized.    Patent Document 1: Japanese Unexamined Patent Publication No. 11-219022, Official Report
In the developing unit which uses a two component developer, optical density unevenness called sleeve memory may occur on an image.
In the development with the two component developer, toner supported on a developing sleeve shifts in a developing region onto the surface of a photoreceptor, and develops a latent image on the photoreceptor so as to form a toner image. Then, on the surface of the developing sleeve having passed the developing region, toner to develop a subsequent latent image is supported.
The sleeve memory is a defect phenomenon caused by the fact that toner is pressed onto the surface of the developing sleeve in the developing region and adheres on the surface of the developing sleeve. In the developing region, since toner is pressed onto a surface portion of the developing sleeve which faces non-image portions in a latent image on a photoreceptor, toner tends to adhere on there. On the other hand, for example, since toner is not pressed onto a surface portion of the developing sleeve which faces image portions in the latent image, toner is unlikely to adhere on there.
Although the developing sleeve having passed over the developing region supports toner for developing a subsequent latent image, the height of a toner layer on a surface portion of the developing sleeve on where toner adheres is higher than that of a toner layer on a surface portion on where toner adheres. Further, a difference in electric potential between image portions of the photoreceptor and the surface of the developing sleeve is larger at the portion on where toner adheres than at the portion on where toner does not adhere. Accordingly, when latent images to be developed with the same optical density are developed, the optical image density developed with the portion where toner adheres becomes higher than the optical image density developed with the portion where toner does not adhere.
Therefore, a phenomenon that the optical density of an image when a subsequent latent image was developed with the portion of the developing sleeve which faces a non-image portion in a latent image in the developing region and on where toner adheres, becomes higher than the optical density of an image when a subsequent latent image was developed with the portion (on where toner is unlikely to adhere) which faces an image portion, may occur. Sleeve memory means such a phenomenon.
The unevenness of the toner adhering on the developing sleeve may be removed by a layer thickness regulating member that regulates the thickness of a conveyed developer layer, i.e., a layer thickness, or a scraper to rub with sliding movement a developer layer on the surface of the developing sleeve, and the removal of the unevenness of toner prevents the occurrence of the sleeve memory. However, when being subjected to regulating of a layer thickness or rubbing with sliding movement, toner receives stress. Such stress causes lowering of charging ability of toner and lowering of fluidity, which results in lowering of the developing efficiency.
The developing unit disclosed by Patent Document 1 comprises a first developing roller equivalent to an upstream side developing roller and a second developing roller equivalent to a downstream side developing roller. Further, the developing unit is provided with a blade to regulate a layer thickness of developer conveyed by the first developing roller, and a scraper to scrape off developer from the second developing roller, so that the sleeve memory may be prevented. However, it is unavoidable that toner receives stress from the layer thickness regulating member and the scraper.
In the developing unit which is equipped with two or more developing rollers and a developer is handed over from an upstream side developing roller to a downstream side developing roller, agitated developer is supported and conveyed by a developing roller, i.e., an upstream side developing roller, and the layer thickness of toner supported on the upstream side developing roller is regulated by a layer thickness regulating member so as to make adhering toner drop of whereby the occurrence of the sleeve memory may be prevented. However, it is unavoidable that toner receives stress. Therefore, in order not to increase stress applied to toner, for example, it may be preferable not to arrange a layer thickness regulating member or a scraper around the downstream side developing roller. However, with this structure, the sleeve memory tends to occur.